Video projectors have been used extensively in business environments and have recently come into wide use in large-screen projection systems in home theaters. The miniaturization of projection systems has led to the development of “pico-projectors” that may be embedded in other systems, such as mobile phones and heads-up displays for vehicle dashboards, or may be implemented as stand-alone devices, such as pocket or ultra-mobile projectors that may be powered from a battery or an external power source. These miniature systems use highly efficient LED or LASER light sources.
One example of a pico-projector system is the PicoP™ projector engine developed by Microvision, Inc. The PicoP engine includes RGB laser sources, a micro-electro-mechanical system (MEMS) scanning mirror, optics and video processing electronics for receiving video data from a source and generating an image to be projected onto any viewing surface (e.g., a screen, a wall, a sheet of paper or a chair back). However, projection systems that use a MEMS scanning mirror face some unique technical problems that are not evident with other methodologies.
A conventional MEMS scanning mirror implemented in a pico-projection system is a two-dimensional scanning mirror that sweeps laser beams across a viewing surface similar to the vertical and horizontal sweep of an electron beam in a CRT-based television or monitor. The horizontal sweep is typically done at one of the resonant mode frequencies of the scanning mirror that is on the order of 18 kHz. Operating on a resonant mode allows maximum beam deflection with minimal input energy. Although the horizontal movement is sinusoidal, the image may be pre-warped by an image processor in order to compensate for the sinusoidal movement. The vertical sweep is generally desired to be an ideal saw tooth to provide a linear sweep movement from top-to-bottom with minimal retrace time, thus maximizing the allowable active video time. A typical MEMS may have a horizontal and vertical drive input and horizontal and vertical sensor outputs. Each sensor provides an electrical signal in proportion to mirror movement in that axis. In that way, the actual movement and/or position of the scanning mirror can be monitored and/or controlled.
Ideally, the MEMS scanning mirror would have only one resonant mode at the horizontal sweep frequency. As described, the resonant mode associated with horizontal sweep is beneficial. Unfortunately, the mirror has multiple resonant modes for both vertical and horizontal movement. The first mode typically falls inconveniently within the frequency spectrum occupied by the vertical drive.
While the horizontal drive is a narrow band signal, a sine wave, falling right on a resonant mode, the ideal vertical waveform is a saw tooth or a modified saw tooth. This wideband waveform has harmonics extending to over 1 kHz, including some which will inevitably fall on or near the first mode of the MEMS scanner.
The high Q of the MEMS at its first mode will cause an accentuation of nearby frequency components about its vertical axis motion. This will distort the vertical sweep waveform, resulting in visible distortion of the raster image. To suppress these first mode oscillations in the vertical axis, various filtering methods such as low pass filters and/or notch filters may be employed. Similarly, the waveform may be created from a lookup table which is created with low pass and/or notch response.
Low pass and/or notch filtering may be employed with some degree of success. However, each has specific problems that are difficult to overcome, especially in a high volume manufacturing environment. Low pass filtering with adequate attenuation of the first mode may distort the vertical sweep waveform causing visible distortion at the top and/or bottom of the raster. Alternatively, the number of horizontal sweep lines may be reduced to avoid displaying this distorted region. Either result is undesirable.
Notch filtering has the ability to virtually eliminate first mode oscillations. Unfortunately, if the filter center frequency is even 1% different from the individual MEMS first mode frequency, unacceptable visible distortion of the raster image due to first mode oscillation will result. This makes implementation of a fixed frequency notch filter impractical in a high volume manufacturing environment.